In order to provide for durable, low cost, highly accurate absolute pressure sensors, it is desirable to utilize modern semiconductor processing techniques for machining the critical parts of the sensor, rather than to manufacture the parts separately as taught in the prior art.
Guckel et al. in U.S. Pat. No. 4,744,863 discloses the use of a sealed cavity semiconductor pressure transducer employing a flexible, polysilicon deformable diaphragm that is formed over a sacrificial oxide post on a semiconductor substrate. The sacrificial post includes a plurality of weblike appendages extending along the substrate beyond the periphery of the post. After the diaphragm material is conformally deposited on the post, the periphery of the diaphragm material is etched away to reveal the sacrificial oxide. The sacrificial oxide is then etched away beneath the flexible diaphragm in order to define a diaphragm cavity. The holes formed in the circumference of the diaphragm from the web appendages that have been etched away are then sealed in order to form the diaphragm cavity.
The preferred embodiment uses piezoresistive current conducting components that are deposited on the flexible diaphragm. The resistance of these deposited materials will change in proportion to the deformation of the flexible diaphragm as the ambient pressure changes with respect to the pressure sealed within the diaphragm cavity. At column 6, at lines 47-63, mention is also made of using the structure for carrying two plates of a capacitive sensor such that the capacitance will change in response to the deflection of the diaphragm, but no capacitive sensor embodiments or teachings are provided.
In contrast to the prior art, the present invention utilizes a polysilicon diaphragm that is formed on top of a sacrificial oxide that was previously deposited on a silicon substrate, where the thickness of the sacrificial oxide layer is generally equal to the thickness of the diaphragm cavity to be formed. The use of a highly controllable silicon dioxide sacrificial layer on the silicon wafer yields a capacitor having a very small capacitance and minimum parasitic capacitance. The fabrication process does not require the bonding of patterned structures for the diaphragm. The manufacturing process can be controlled such that very low cost sensors can be manufactured without calibration steps.